1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to an optical transmission system with a mechanism to locate a fault on an optical transmission line.
2. Description of the Related Art
The international telecommunication needs are rapidly growing in recent years, the major driving forces being business globalization and Internet proliferation. In such circumstances, submarine optical transmission systems are expected to be as important as satellite communications systems, to meet the increasing demands for more cost-effective, bandwidth-rich telecommunication service.
In submarine optical transmission systems, in-line repeaters are placed at certain intervals along the fiber optic cable laid under the sea, so that the attenuation losses of signals will be compensated for by intermediary optical amplification. Such submarine systems are required to offer the highest level of reliability because a trouble in the undersea equipment would cost a lot of time and money to repair. For this reason, the system must have an integral troubleshooting mechanism that pinpoints the fault in the event of such a failure.
Conventional fault locating techniques use a probing light signal that is transmitted from an end station on the land, so that the light will travel through the fiber optic links and repeaters until it is reflected at the point of a fiber break. By measuring the reflected light, the end station identifies the location of the fault. One problem with such conventional technique is that the probing light signal would be degraded before it reaches the broken point, because the light has to pass through a number of optical couplers in the repeaters. The worsened signal-to-noise ratio (SNR) would reduce the accuracy of fault location measurement, as well as increasing the time for analysis.
To solve the above problem, the researchers have proposed other fault locating techniques in which the probing light is transmitted by an undersea repeater. For example, the Unexamined Japanese Patent Publication No. 4-326218 (1992) proposes a system with an extra fiber optic cable dedicated for transporting a probing light signal, although the cost of the system would increase because of the additional undersea cable laid in parallel with the main signal cable. The Unexamined Japanese Patent Publication No. 6-268597 (1994) discloses a technique which serves the same purpose, but is different from the foregoing system in that the fiber optic cable for main communication channels is used to measure a reflected light.
The above-described conventional troubleshooting techniques, however, can only detect a fiber break, because the object of their measurement is confined to Fresnel reflected lights. In other words, the conventional techniques are unable to cover the other types of faults that would not cause Fresnel reflection. Not only being disrupted by a fiber break, optical transmission signals may also be degraded by increased fiber losses due to the diffusion of hydrogen or variations in temperature. Note that those kinds of fiber deterioration would never be observed as a Fresnel-reflected light. Trouble within a repeater is another failure mode that must be taken into consideration. It is therefore necessary to develop an enhanced fault locating system that covers various types of faults, including fiber deterioration and repeater failure, besides being capable of detecting a fiber break.
Yet another requirement for the system is a capability of controlling a repeater from a distant end station effectively and efficiently to make the repeater transmit a probing light signal, which none of the conventional proposals have offered. This requirement for the quality of interruption between an end station and repeaters has to be fulfilled in the future optical transmission systems.
In view of the foregoing, it is an object of the present invention to provide an optical transmission system which has a capability of locating a fault on a transmission line effectively and efficiently to ensure the quality of communication between end stations and repeaters.
To accomplish the above object, according to the present invention, there is provided an optical transmission system with a mechanism to locate a fault on an optical transmission line. This system comprises: repeaters which relay optical signals over the optical transmission line, and an end station which controls optical signal transmission and remotely manages the repeaters.
Each repeater comprises monitoring report signal generating unit and a light pulse signal sending unit. In response to a monitoring control command sent from the end station, the monitoring report signal generating unit monitors the current operating status and input/output signal conditions of the repeater itself, and it generates a monitoring report signal to inform the end station of the monitoring results. In response to a troubleshooting control command sent from the end station, the light pulse signal sending unit transmits a probing light pulse signal to the optical transmission line to locate a fault thereon, as well as sending a complementary light pulse signal that is complementary to the probing light pulse signal.
The end station, on the other hand, is equipped with a monitoring controller and a troubleshooting unit. The monitoring controller comprises: a monitoring command sending unit which sends a monitoring control command to the repeaters; and a monitoring report processor which monitors the operating status of the repeaters by analyzing the monitoring report signals received therefrom, as well as identifies a faulty link section if any problem with the optical transmission line is detected. The troubleshooting unit comprises: a troubleshooting command sending unit, a Rayleigh backscatter measurement unit, and a fault detection unit. The troubleshooting command sending unit sends a troubleshooting control command to one of the repeaters that is located near to the identified faulty link section. The Rayleigh backscatter measurement unit measures a Rayleigh backscattered light caused by the probing light pulse signal, using the complementary light pulse signal as a reference for synchronization. By analyzing the measured Rayleigh backscattered light, the fault detection unit locates the fault on the faulty link section.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.